The present invention pertains to the art of measuring electric and magnetic fields in a conducting medium. In particular, the invention applies to a system and method to determine the electrical resistivity of the seafloor, and most particularly to offshore hydrocarbon exploration.
Various techniques require measurement of electric and magnetic fields underwater and in particular at the sea floor. The measurement of magnetotelluric fields and controlled-source electromagnetic (CSEM) sounding are examples. Present systems to perform these measurements are comprised of a series of independent sensors and data recording modules connected to a common frame. An example of a magnetotelluric measurement system built according to U.S. Pat. No. 5,770,945, incorporated herein by reference, and used in commercial hydrocarbon exploration, is shown in FIG. 1.
As illustrated, the magnetotelluric measurement system for seafloor oil exploration is roughly grouped into four units. A first unit, a logger unit, includes a 4-channel digital data logging processor 104, magnetic field post amplifier, electric field amplifiers, all contained within a first waterproof pressure case 112. A second unit is a second waterproof pressure case 114 containing an acoustic navigation/release system 116. A third unit consists of four silver/silver-chloride (Ag—AgCl) electrodes 118-121 mounted on the ends of 5-meter long booms 139-142. A fourth unit includes magnetic induction coil sensors 122, 124. (Note that a third magnetic sensor can be used to measure the vertical magnetic field, but is not illustrated.) Each of the elements of the system are mounted on a corrosion-resistant plastic and aluminum frame 102 along with glass spheres 126 for flotation and an anchor 128 for deployment to the seafloor.
The size and operational methodology of prior art underwater EM measurement systems such as the one shown in FIG. 1 have been driven by: a) the need for a large separation between the electrodes used in the electric field sensors; and b) the need to store the electrodes in water prior to deployment into the ocean. The result is a cumbersome system with sensing arms than span 10 meters and sensors that must be installed into the system on the deck of a ship, just prior to deployment. Putting together so sensitive a system on the deck of a ship is a difficult task that reduces the overall reliability of the measurement system when deployed. In particular, there are a number of discrete modules interfaced by connectors that have the potential to leak and be exposed to seawater, thereby raising the risk of corrosion potentials in the sensing circuit, and in general there is a risk of damaging connector pins.
The fundamental limitation in present underwater electrodes is their requirement to exchange ions with the seawater in order to provide a real (i.e. DC) electrical current path between a first stage amplifier and the ocean. The actual process by which current passes from an electrode into the medium can be complex, involving direct tunneling of electrons between the electrode and medium, chemical reaction to transfer electrons from/to ions in the medium, and catalysis of chemical reaction in the medium at the electrode surface and associated charge transfer to the electrode. These processes are typically accompanied by the diffusion and field-induced motion of ions in the medium to carry the current away from the electrode. In this document, such current conduction mechanisms are referred to as “resistive,” though the actual process is much more complex, and will often not conform to Ohm's law. For a resistive current to flow, the medium must be a fluid or solid environment which can couple to an electrode in a resistive manner, no matter how weakly, such as seawater, soil, or suitable compliant rock (e.g. sand).
These chemical reactions and associated diffusion-driven effects mean that resistively coupled electrodes have an inherent settling time and associated level of low frequency electrical noise. The ionic boundary layers and concentration gradients typically take times on the order of 10 minutes to reach equilibrium. In addition, there is a small DC potential difference associated with this equilibrium that depends on the manufacture and usage history of the electrode. Small variations in this DC potential due to effects such as temperature changes or variations in the local chemical environment (e.g. salinity) lead to an increase in the electrode measurement noise at low frequency. In a shallow water environment, the inevitable stirring of the solution will impede this equilibrium, thereby changing the DC potential and increasing noise. In order to minimize these effects, resistive electrodes are generally made as large as practical so that their electrical resistance to the medium is as small as possible. However, even at their largest practical size (˜30 cm long), the noise from electrodes is often the limiting factor in the system performance. Further, the chemical processes mean that existing resistive electrodes must remain wet at all times to function at their optimum level.
In other applications, electrodes that do not require coupling to the medium in a resistive manner have been developed. Such electrodes have a primarily capacitive interaction with the medium, in which they couple directly to the electric potential at a given point in the medium via the rate of change of the local electric flux density. In capacitive coupling, an image charge flows to the electrode to neutralize its electrostatic energy relative to the medium and this charge creates an equal and opposite charge in the input of the readout apparatus. The coupling is primarily capacitive because in practice there is no perfect electrical insulator, and in contacting any medium a small, but non zero resistive current will flow.
The majority of primarily capacitive electrodes have been developed to measure signals that originate in the human body, or are implanted in order to apply signals to stimulate the body. An example of the former is capacitive electrodes that work on the skin and/or just off the skin. The aim of such measurements is to avoid the use of the conducting gels that are used in conjunction with resistive electrodes in order to reduce the impedance of the outer layer of the skin from a resistance greater than 100 kΩ to less than 10 kΩ. The capacitance of skin contacting electrodes is on the order of 0.1-100 nF, resulting in a system noise with a well designed amplifier on the order of 1 μVrms. This noise level is comparable to the skin noise of the human body and is adequate to record cardiac and brain signals.
In the latter case of implanted stimulating electrodes, the concern is to prevent a DC current flow which could lead to a build-up of electrolysis products that can be toxic to tissue. Traditionally, a capacitor is placed in series with a resistive electrode in order to block such DC current. To reduce the overall system size, a capacitive electrode is sometimes used to remove the need for the series capacitor. To apply a stimulus pulse to the cerebral cortex or to the cochlea requires current pulses on the order of 5 mA for a duration on the order of 1 millisecond. This can easily be provided by an electrode with capacitance to the medium on the order of 1 μF, and an applied voltage on the order of 5 V. When properly constructed, a capacitive stimulating electrode of this type has a size on the order of 1 mm in diameter and 0.25 mm in thickness. In some applications, the stimulating electrodes are used to receive signals in the range 100 μV in amplitude and 2 to 70 Hz in frequency that are indicative of electromechanical body activity such as tremors, akinesia and rigidity that might signify a modification in treatment is needed. However, at 1 Hz the impedance of a stimulating electrode of this type is on the order of 200 kΩ. When used with a state-of-the-art preamplifier with input current noise of 1 pA, such an electrode has an rms voltage noise over the range 2 Hz to 70 Hz of at least 3 μV, which is on the order of 100 times higher than existing underwater resistive-based electrode measurement systems.
A recently developed type of sensor makes capacitive coupling to the air. In this case it is not possible to make a usable resistive contact to the air and a capacitive coupling is the only option. However, the achievable capacitance to the air is very small, on the order of 1 pF (i.e. 1000 times less than for skin contacting electrodes and 1 million times smaller than for stimulating electrodes). The resulting noise level at 1 Hz is on the order of 100 μV, again far higher than existing underwater resistive-based electrode measurement systems.
Thus, although capacitive electrodes offer a way to measure electrical potentials without electrochemical reactions with the medium, prior capacitive electrodes systems have been considerably noisier than what can be achieved by existing underwater resistive electrodes. Further, even if all noise associated with present capacitive coupling could be removed, there would still be the possibility of electrochemical noise with a nominally capacitive electrode. Indeed, because of their comparably high internal noise, the presence of electrochemical noise between the electrode and the medium has not previously been a design consideration in capacitive electric field measurement systems.
Based on the above, there exists a need for a compact underwater electromagnetic measurement system that can be stored on a ship and moved into the water in a fully assembled form without disconnecting or adding sensor elements. Further, there exists a need for a measurement system that can confirm its general functionality prior to deployment into the ocean, and then confirm to a high level of accuracy its full operating performance when on the sea floor. One application of significant commercial and research interest is the field of underwater magnetotellurics and CSEM sounding, in which electric and magnetic field measurements are made at a number of locations, allowing the conductivity of the underlying geology to be inferred.